What dictates the size of a star?

[Kurious12]

Compared to the size of the Earth the sun is massive but there are stars out there that would reach out to the orbit of Jupiter if they were where our sun is. I read that when our star reached a certain size it started fusion at the core and then stopped growing because it will start to blow all the gases away that it was feeding on. Most stars are born in a cloud of gas and dust that can be light years across so there's no end of material to feed on while growing. With that in mind what determines how small or large a star will become during its birth?

[beecee]

34 minutes ago, Kurious12 said:

Compared to the size of the Earth the sun is massive but there are stars out there that would reach out to the orbit of Jupiter if they were where our sun is. I read that when our star reached a certain size it started fusion at the core and then stopped growing because it will start to blow all the gases away that it was feeding on. Most stars are born in a cloud of gas and dust that can be light years across so there's no end of material to feed on while growing. With that in mind what determines how small or large a star will become during its birth?

A star essentially is a balancing act between radiative pressure trying to expand the star, and gravity compressing it. These essentialy are determined by the nuclear fusion processes going on in the core of the star. Consequently, a star’s size is set by its own mass. 

Also (although I am less certain as a non physicist) the original first generation stars were much larger in volume, with consequently shorter life spans. These stars formed from the accretion of large areas of H and He with no metallic content. So I would also hazzard a guess and say the content of the cloud from whence the stars formed. Later generation of stars are smaller and obviously formed from clouds of dust and debris, that did have metallic content, that was synthesised from the original larger stars.

I would wait though for some confirmation on that.

[Phi for All]

21 minutes ago, Kurious12 said:

Most stars are born in a cloud of gas and dust that can be light years across so there's no end of material to feed on while growing.

It's a mistake to say "there's no end of material to feed on" when the material has a limit ("light years across"). As beecee says, what dictates the size of a star is its mass, or the dust and gases it accumulates as it forms. 

[Genady]

39 minutes ago, Phi for All said:

It's a mistake to say "there's no end of material to feed on" when the material has a limit ("light years across"). As beecee says, what dictates the size of a star is its mass, or the dust and gases it accumulates as it forms. 

Also, their chemical composition or metallicity.

[beecee]

2 hours ago, Genady said:

Also, their chemical composition or metallicity.

that would validate then what I was trying to say here....

3 hours ago, beecee said:

Also (although I am less certain as a non physicist) the original first generation stars were much larger in volume, with consequently shorter life spans. These stars formed from the accretion of large areas of H and He with no metallic content. So I would also hazzard a guess and say the content of the cloud from whence the stars formed. Later generation of stars are smaller and obviously formed from clouds of dust and debris, that did have metallic content, that was synthesised from the original larger stars.

I would wait though for some confirmation on that.

 

[Genady]

7 minutes ago, beecee said:

that would validate then what I was trying to say here....

 

Sure.

Another limitation is that if an original cloud is too large, it would break and make several smaller stars rather than one large star.

[beecee]

3 hours ago, Genady said:

Sure.

Another limitation is that if an original cloud is too large, it would break and make several smaller stars rather than one large star.

A reason in fact why our own star the Sun, is not part of a binary, trinary or bigger system...actually relatively rare in that respect.

[Kurious12]

Cool, so the composition of the cloud it forms from plays a role in its formation.... thanks

[Airbrush]

The SIZE of a star is determined more by how old it is than its' mass.  Very massive stars are not so giant when they are young.  For example, our sun will grow in size until it reaches Earth's orbit.  Very massive stars burn hot and die young in a supernova, before they grow much in size.

Edited May 9, 2022 by Airbrush

[Intoscience]

As already been said the size of a star is determined by its mass and its age. There are a few considerations to think about when we talk about "size", there is the minimum mass it takes to form a star in the first instance, then there is the total amount of mass distribution available along with other factors that determine the eventual mass and then there are the cycles a star goes through during its life. So the mass changes and also the diameter changes during a star's life span. For example, in the next 4-5 billion years its estimated that our own sun will grow in diameter, possibly expanding so far it comes close to, maybe reaching the Earth's orbit. 

Here is a link to the life of a star - https://en.wikipedia.org/wiki/Star

 

Edited May 10, 2022 by Intoscience

[MigL]

If stars were composed of purely Hydrogen then all stars would be the same size, and limited only by availability of the gas.
You could have small, unburning Jupiters, but at a certain mass, all H₂ composed stars will start burning ( fusion ).

Adding heavier elements to the mix, like Helium, will delay the onset of fusion and allow the star to become larger because Helium will sink to the core and burning starts at much higher temperatures/pressures.
Carbon and other heavier cores will start even later.
That is the reason our Sun will swell to a red giant in its old age; the build up of 'heavy' elements at its core.

Genady mentioned metallicity, the amounts of even heavier elements present in second generation stars, which may act as moderators in the fusion process, and keep it from starting early.

These factors, and probably others I haven't thought of, all play a role in the onset of fusion, which then blows the rest of the gas away.

Edited July 12, 2022 by MigL

[swansont]

9 hours ago, MigL said:

These factors, and probably others I haven't thought of, all play a role in the onset of fusion, which then blows the rest of the gas away.

But it doesn't blow the rest of the gas away. For example, our sun has much more mass in it than just the core, which has only about 10% of the sun's mass in it.

Radiation pressure is relatively weak; you need a large photon scattering rate to cause appreciable acceleration. So you will blow gas away, but it depends on the luminosity of the star and what the gravitational acceleration is. At some luminosity you blow gas away at some distance from the star because gravity drops off, but gas inside that point still feels a net attraction.

"The Eddington limit is the point beyond which a star ought to push itself apart, or at least shed enough mass to reduce its internal energy generation to a lower, maintainable rate. The actual limit-point mass depends on how opaque the gas in the star is, and metal-rich Population I stars have lower mass limits than metal-poor Population II stars."

https://en.wikipedia.org/wiki/List_of_most_massive_stars#Eddington_mass_limit

[MigL]

Thanks Swansont.
The Eddington limit is one other such factor, which I didn't remember.

[swansont]

30 minutes ago, MigL said:

Thanks Swansont.
The Eddington limit is one other such factor, which I didn't remember.

Been there many times. It's always so interesting to dive into these scenarios. Even when the basic description covers most of what's going on, the rabbit hole always goes deeper.